The invention relates to a method of and apparatus for processing a binary, ternary or higher order compound to improve stoichiometry. The invention further relates to fabricating a device, and to such devices.
Physical vapour transport (PVT) is a crystal growth technique used for preparing CdTe. It has been recognised that PVT of CdTe requires a highly stoichiometric starting compound, since the transport rate of the vapour species falls off rapidly as the composition of the starting compound deviates from perfect stoichiometry (see reference [1] & reference [6]). If the starting compound has perfect stoichiometry, i.e. equal amounts of the species Cd and Te, then congruent sublimation of the vapour takes place. However, as the stoichiometry deviates to either excess Cd or excess Te, the transport rates fall by orders of magnitude. If the deviation is too large, growth is completely inhibited.
The use of highly stoichiometric starting material is beneficial not only for PVT growth of CdTe but also for other materials and growth techniques, such as Bridgman, high-pressure Bridgman and Markov.
Su et al (see reference [2]) heat treat a CdTe starting compound to remove excess Te and improve the CdTe stoichiometry. Samples were heat treated, i.e. annealed, for 24 or 48 hours at 870° C. To examine the effect of the heat treatment, the CdTe stoichiometry was measured before and after heat treatment using an optical absorption technique. The optical absorption technique is described in Carles et al [3] and Mullins et al [4] where the technique is used during vapour growth. The optical absorption is used to measure the partial pressure of Te2 vapour, P(Te2), while the sample is maintained at an elevated temperature of 870° C. The corresponding partial pressure of Cd, P(Cd), is calculated from the stoichiometric invariant, i.e. the equationP(Cd)P(Te2)1/2=K(T)where K(T) is the equilibrium constant which depends only on temperature T. (The Cd pressure was too low to measure accurately the Cd absorption peak in the Te-rich samples used). The partial pressure ratio P(Cd)/P(Te2) is a measurement of the stoichiometry of the CdTe. If the CdTe has perfect stoichiometry, then material loss is solely by congruent sublimation which implies loss of equal amounts of Cd and Te thereby giving P(Cd)/P(Te2)=2. Other Cd and Te vapour species are only present in small amounts at 870° C. and are ignored.
The optical absorption measurements showed that the as-synthesised material was in all cases Te-rich. P(Cd)/P(Te2) varied between 0.007 (highly Te rich) to 1.92 (only very slightly Te rich). After heat treatment, measured P(Cd)/P(Te2) varied between 1.84 and 3.4. In particular, the heat treatment was successful in removing the large quantities of excess Te from the highly Te-rich samples, providing material that generally has moderate excess amounts of Cd.
Heat treatment is thus shown generally to improve stoichiometry of the Te-rich as-synthesised material, although precise control of the stoichiometry is not possible. An unwanted consequence of the heat treatment is material loss of the CdTe compound. The 24 or 48 hour anneal was performed under vacuum and the material loss stated to be ‘reasonable’. But, during the pre-anneal and post-anneal optical absorption measurements, which are performed under non-vacuum conditions, the required heating to 870° C. is reported to result in loss of about 10% of the CdTe in only 8 minutes. It is not possible to measure the stoichiometry during the vacuum anneal because the vapour pressures are too low.
Further heat treatment methods for CdTe are discussed by Giebel et al [8] and by Mochizuki [9].